Probing Polaron Environment in a Doped Polymer via the Photoinduced Stark Effect

Conjugated polymers (CPs) constitute a unique class ofmaterialswith applications ranging from energy storage to bioelectronics, thanksin part to their ability to be easily doped to produce mobile chargecarriers. While methods for doping CPs are well-developed, a deeperunderstanding of the interplay between the local environment and polaronicproperties is currently lacking. Using transient absorption (TA) spectroscopy,we demonstrate that photoexciting polarons in a lightly doped 3,4-propylenedioxythiophene-co-3,4-ethylenedioxythiophene (ProDOT-co-EDOT) polymer film produces a Stark effect that is sensitive totheir local environments. The resulting electroabsorption (EA) signal,which originates from a shift in the band gap of nearby polymer chainsdue to the photoinduced electric field, indicates that polaron excitationspatially separates the hole from its counterion as the hole thermalizeswithin 200 fs. Changes in the spectral position of the EA signal whentuning the excitation wavelength across the P1 and P2 polaron bands,combined with a dynamic blue shift, suggest a variation in field strengththat senses different extents of the local environment. We proposethe photoinduced Stark effect in TA experiments as a sensitive probeof the local environment of polarons in doped CPs that can also beused to refine electronic structure models describing their polaronicstates and transitions.